This invention relates to an attachment lens inserted between an objective lens and a camera body to increase the focal length of the objective lens.
The use of a lens of negative focal length attached between an interchangeable objective lens and a film plane to increase the focal length of the objective lens has been widely accepted in the single lens reflex camera art, as is known, for example, in U.S. Pat. No. 4,154,508. The assignee of the present invention has proposed an attachment lens in Japanese Patent Publication No. Sho 51-12421, and, later on, this attachment lens after having been expanded to improve the aberrational performance is described in U.S. Pat. No. 4,129,359.
However, whilst such an attachment lens at the rear of the objective lens has merit in that it serves not only to increase the focal length with ease, but also to call for little increase in the bulk and size, it entails the disadvantage that the F-number at full open aperture is increased, that is, the resultant objective system becomes darker. For example, suppose the master lens has a focal length of 50 mm with F-number at full open aperture of 1:2, and the rear attachment lens has a magnifying power of two times, the resultant objective system is simply found to have a focal length of 100 mm with a F-number of 1:4. As the presently most common standard lens is provided with focal length of 50 mm and F 1:1.4, therefore, the usefulness of the 2.times. rear attachment lens in application to this master lens can be assured when the resultant F-number at full open aperture 1:2.8 is generally regarded as an acceptable value for long lenses.
Ordinary photographic objectives are already corrected for the various aberrations. It is desirable to preserve their high grade imaging performance even after the attachment lens is mounted. To fulfill this requirement, a good aberrational correction of the rear attachment lens itself must be achieved.
However, the rear attachment lens, because of its focal length being negative, has the property of producing a Petzval sum of negative sign. As a counter-measure against this, the lens configuration must be selected so that the front principal point of the rear attachment lens lies as near the master lens as possible, whereby while allowing for an axial separation between the master lens and the attachment lens, the interval between the principal points of these lenses can be shortened to permit the refractive power of the attachment lens to be reduced with the magnifying power remaining unchanged. As a result, the Petzval sum is decreased in the negative sense to prevent deterioration of the curvature of field (astigmatism). A further reduction of the Petzval sum in the negative sense can be achieved by selecting an optical glass of high refractive index for employment in the positive lens elements, and that of low refractive index for employment in the negative lens elements. In connection with the arrangement of the attachment lens and the master lens, however, it should be noted that the diaphragm in the master lens is effective for the attachment lens. Thus, the principal ray of a pencil of off-axial rays does not cross the optical axis when in passing through the attachment lens, as it is laid in either one of the media on the upper and lower sides of the optical axis. Unlike this, the popular types of lenses have diaphragms that are generally located in a space within the lens, so that the principal ray crosses the optical axis at a point within the lens, and so that the aberrations of the upper part from the optical axis and lower part are cancelled out to some extent. This simplifies the expressions for the off-axial aberrations of the master lens. In the attachment lens, the expressions for the off-axial aberrations take a more complicated form.